United Flight 175 flies toward the south tower of the World Trade Center in New York as the north tower burns after being hit by American Flight 11 a short time earlier, in this file photo from September 11, 2001.

United Flight 175 flies toward the south tower of the World Trade Center in New York as the north tower burns after being hit by American Flight 11 a short time earlier, in this file photo from September 11, 2001. (Sean Adair-Files, Reuters)